The effect of salts used in textile dyeing on microbial decolourisation ofa reactive azo dye

Azo dyes account for 60 to 70% of all textile dyestuffs produced and are th e most common chromophore of reactive textile dyes. Colouration of textile effluents usually can be linked to the presence of water-soluble (reactive) azo dyes. Under anaerobic conditions azo dyes can be used as terminal elec tron accepters during microbial respiration, being reduced and decolourised concurrently with re-oxidation of reduced flavin nucleotides. Thus, the pr ocess of anaerobic digestion is under investigation as an effluent treatmen t option for the textile industry. The ability of anaerobic microbial conso rtia to decolourise azo dyes is well established; however, an additional fa ctor that requires consideration is the high concentration of salts present in reactive dyeing effluent. Nitrate, sulphate, chloride and carbonate sal ts can all be used during reactive dyeing. This paper presents results of a n investigation into the effect of nitrate and sulphate on the decolourisat ion of a reactive azo dye, Reactive Red 141. Nitrate was found to delay the onset of decolourisation for a period of time related to the concentration of nitrate initially present in the system. Sulphate was found to have no discernible effect on the onset or rate of decolourisation and it is propos ed that the order of reduction of the compounds is nitrate > Reactive Red 1 41 > sulphate. Measurement of redox potential in the anaerobic system durin g decolourisation, with or without the addition of nitrate or sulphate, sho wed that strictly anaerobic conditions were conducive to rapid decolourisat ion of the azo dye.